Palisade Giant 's ability is a type of replacement effect. You can tell replacement effects by the use of the words "as", "with", "instead", "prevent", or "skip" somewhere in the rules text. If more than one replacement effect could modify how an event affects a player or object, then the affected player or controller of the affected object decides which one to apply. After this, it's possible the modified event could also be replaced by other replacement effects, so you go through the same process until there's nothing left to apply. The key thing is that a replacement effect can only be applied to an event once, and that includes any modifications by other replacement effects.
If Shock is targeting one of the Palisade Giant s, then the other will redirect the damage to itself, and then the first will redirect that damage back to itself. There are no other unused replacement effects to be applied, so the damage stays there.
If Shock is targeting you, you choose one Palisade Giant to redirect the damage to itself, then the other will redirect the damage to itself. Likewise, there are no other unused replacement effects to be applied after this.
July 31, 2014 2:12 p.m.
Okay so each giant will try to take the damage only once each. That makes sense, thanks for clarifying.